1. Field of the Invention
The present invention relates to a reverse tension mechanism for a strapping machine and, more particularly, to a reverse tension mechanism for retracting and tightening a strapping band for strapping machines.
2. Description of the Related Art
FIGS. 9 to 11 show a conventional strapping machine which includes a forward-reverse assembly 30 and a tension assembly 50. The forward-reverse assembly 30 includes a combination of an active wheel 311 and a passive wheel 312 to perform forwarding and reversing actions of a strapping band 20. The tension assembly 50 includes another combination of an active wheel 511 and a passive wheel 513 to allow the strapping band 20 to extend therebetween and to be tightened. The tension assembly 50 further includes a lever unit 52 having a horizontal rod 53, a tension arm 54, and a driven arm 55. One end of the horizontal rod 53 is pivotably connected to a machine body 10, and the other end of the horizontal rod 53 provided with a rotary member 531 in contact with a cam 42. A spring 56 is installed between the horizontal rod 53 and one end of the tension arm 54. The other end of the tension arm 54 is pivotably connected to one end of the driven arm 55, and the other end of the driven arm 55 is rotatably connected to the machine body 10. Further, the passive wheel 513 is pivotably connected to the driven aim 55 and adjacent to the active wheel 511. The horizontal rod 53 can be actuated by the cam 42, so that the passive wheel 513 is moved to change a position relative to the active wheel 511 when the horizontal rod 53 swings. Accordingly, the tightening force exerted on the strapping band 20 by the active wheel 511 and the passive wheel 513 can be controlled. However, the lever unit 52 composed of the horizontal rod 53, the tension arm 54, and the driven arm 55 occupies a large space horizontally. Thus, both the cost and size of the conventional strapping machine are increased. Furthermore, since the spring 56 is disposed adjacent to the rotary member 531 of the horizontal rod 53, the compressed displacement of the spring 56 is small when the cam 42 activates the horizontal rod 53 to swing (see FIG. 11). Thus, a pushing force transmitted to the tension arm 54 from the wiggling motion of the horizontal rod 53 is weak, and the weak pushing force further transmitted to the passive wheel 513 and the active wheel 511 is incapable for clamping the strapping band 20 tightly. The strapping band 20 will easily slip off from an object which needs strapping.